The need for high-quality, top-speed computer printers and other types of output printers with changeable formats has been evidenced in recent years. Developments have proceeded with respect to ink jet technology to answer this need. Most developments in the field of ink jet have related to pressure deflected systems such as taught by Sweet U.S. Pat. No. 3,596,275, wherein a single stream of ink droplets are selectively charged and passed through a uniform deflection field to impact various locations on a recording medium in accordance with the charge of each droplet. Thus, by applying suitable charging signals to the droplets, a visible human-readable printed record may be formed on the recording surface. This type of system requires very precise control over the charge placed on each droplet due to various factors such as the tendency of similarly charged droplets closely adjacent to one another to repel each other and therefore impact the recording medium at unintended locations. The circuitry required to accomplish this precise control appears to be relatively expensive, especially when duplicated for each jet of a multi-jet printer, which is required to attain truly high speeds.
Another type of ink jet printing has been developed which offers the potential of attaining high-speed, high-quality variable printing without requiring the expensive precision charging control electronic circuitry. This type of printing may be called the binary pressure type and is shown in Sweet et al, U.S. Pat. No. 3,373,437. This type of system generates a plurality of jets in one or more rows, selectively charging drops with a single charge signal for deflection by a constant field to an ink drop catcher. The uncharged drops continue along the original jet stream path to impact a recording medium. The precision control over charging is not required inasmuch as charged drops impact the gutter and not the recording medium. In the absence of selective deflection, the major disadvantage of this type of ink jet printing has been that one nozzle is required for each printing position across the entire dimension of the path to be printed in a single pass. This requires the fabrication of a vast number of nozzle orifices for a single printer. Examples of ink jet heads designed for this type of printing are Beam et al U.S. Pat. No. 3,586,907 and Mathis U.S. Pat. No. 3,701,998. A method for fabrication of orifices with this type of ink jet head is shown in Taylor U.S. Pat. No. 3,655,530. This method involves the electroplating of the interior of predrilled holes until sufficient material has been plated thereon to reduce the orifice diameter to the desired size. This type of fabrication does not appear to lend itself to an extremely closely spaced linear array of orifices.
High quality printing requires that the individual drops and the spots resulting from impact of the drops on the recording medium be sufficiently small and closely spaced so as to be relatively indiscernible as individual drops, but rather discernible only as part of the resultant printed symbol. This may require the printing of 200 or more drops to the linear inch, each spot being approximately less than seven mils in diameter. To achieve this arrangement with a double row head, wherein the orifices of one row are interleaved with respect to the orifices of the other row as shown in above U.S. Pat. No. 3,701,998, would require orifices no larger than two mils in diameter to be spaced no wider than ten mils from center to center along a single row.
An object of the present invention is to provide an ink jet nozzle structure having an extremely closely spaced array of small orifices.
Another object of the present invention is to provide a method for making an ink jet nozzle structure having a closely spaced array of small orifices.
A major difficulty involved is the fact that the ink or fluid to form the jet must be pressurized and forced through the orifices at relatively high velocities. Any ink jet nozzle structure must therefore be constructed to withstand such pressure and velocities over a long period of time without significant wear or cracking over an extended period of time.